*{
    margin: 0;
    padding: 0;
}

.box{
    display: flex;
}

.left, .right{
    height: 100vh;
}

.left{
    width: 200px;
    background-color: yellow;
    position: relative;
    z-index: 99;
}

.right{
    flex: 1;
    background-color: skyblue;
    position: relative;
    z-index: 1;
}